An Overview of Metalized Ceramics

What is Metalized Ceramic

Metalized Ceramics refers to a layer of metal film deposited onto the specific surface of Engineered Ceramics and then cured in a high-temperature reduction atmosphere (hydrogen or nitrogen) furnace so that the metal film will tightly attach to the surface of the Ceramic Components,  refer to figure 1. After the metalizing process, the ceramic surface offers the characteristics of metal and can achieve a compelling connection between ceramic and metal by employing brazing.

Figure 1: Metalized Ceramics

The Purpose of Ceramic Metallization

As a typical inorganic non-metallic material, Advanced Ceramics have been widely used in various high voltage, high current, and high-pressure vacuum devices because of their excellent electrical, physical & chemical, mechanical, thermal, and optical properties. These practical applications often involve the joint of ceramics and metal parts in different materials, such as stainless steel, oxygen-free copper, Kovar, and so on. 

Since the thermal expansion coefficient of ceramic and metal materials has vast differences, the two materials naturally have a poor wetting effect. In these fields, the sealing surface of ceramic and metal parts has strict sealing strength (tensile strength) and air tightness requirements after brazing. Thus, they can not be directly connected. So, ceramic metallization technology was born.

Strengths of Metalized Ceramics

1. High thermal conductivity—The heat the chip generates can directly transfer to the Ceramic parts  

2. Ideal thermal expansion coefficient—The thermal expansion coefficient of advanced ceramics and chips is similar, and it will not cause too much deformation when the temperature difference changes.

3. Low dielectric constant—The dielectric constant of the ceramic material itself reduces the signal loss, so Technical ceramic materials are widely used in communication equipment and signal transmission.

4. High bonding force - High bonding strength of metal layer and Ceramic Substrate of ceramic circuit board products, up to 45MPa (more remarkable than the strength of 1mm thick ceramic parts themselves)

5. High operating temperature—Ceramics can withstand high and low-temperature cycles with large fluctuations and can even operate at a high operating temperature of 800 degrees for a long time.

6. High electrical insulation—Industrial ceramics are insulating materials that can withstand high breakdown voltages, especially Ceramic Insulators after glazing, and can even be applied in fields with voltages above 100KV.

7. Chemical stability—The ceramic body has better chemical stability. It will not react with most strong acids and bases and will not be oxidized in a high-temperature environment.

Mechanism of Ceramic Metallization

The mechanism of ceramic metallization takes advantage of the different chemical reactions and diffusion migration of various substances in advanced ceramics and metalized layers at different sintering stages, such as oxides and nonmetallic oxides.

As the temperature rises, the liquid phase is formed when all substances react to form intermediate compounds and reach the common melting point. The liquid glass phase has a specific viscosity and produces a plastic flow simultaneously. Afterward, the glass particles are rearranged under the action of capillaries, and the atoms or molecules are diffused and migrated under the drive of surface energy. The pores gradually shrink and disappear with the increase of grain size, thus realizing the densification of the metalized layer.

Ceramic Metallization Methods 

1. Mo-Mn Method

The MO-Mn method is based on refractory metal powder Mo, and then dope a small amount of low-melting point Mn metallization formula, adding a binder coating to the Al2O3 ceramic surface, and then sintering to form a Mo Mn metallization layer.  

2. Activated Mo-Mn Method

The activated Mo-Mn method is an improvement based on the traditional one. The main directions for improvement are adding activators and replacing metal powder with molybdenum and manganese oxides or salts. Both of these improvements are designed to reduce the metalizing temperature. 

3. Silver Paste Sintering Method

The silver method involves applying a layer of Ag paste on the ceramic surface, composed of Ag salt flux and adhesive, and then sintering at high temperatures to reduce Ag ions to elemental Ag. The Ag layer can be reduced by triethanolamine silver carbonate or by adding silver nitrate to ammonia and then reduced by formaldehyde or formic acid.

Due to the strong diffusion of silver ions, the silver paste sintering method is not appropriate for electrical appliances used in strong electric fields. The electrical properties will deteriorate rapidly under high temperatures, high humidity, and direct current electric fields.

4. Active Metal Brazing- AMB

Active metal brazing is also a more widely used ceramic-to-metal sealing process; it is 10 years later than the development of the Mo-Mn method, characterized by fewer processes, shorter cycles, good welding reliability, and suitable for a variety of different ceramic materials. The ceramic-metal sealing can be completed with only one heating process. Brazing alloys contain added Ti, Zr, Hf, and Ta active elements; the added active elements react with Al2O3 to form a reaction layer with metal characteristics at the interface; this method can be easily adapted to large-scale production, compared with molybdenum- - manganese process, this method is relatively simple and economical.

5. Direct Bond Cooper - DBC

DBC is a metallization method of bonding copper foil on a ceramic surface (mainly Al2O3 and AlN), which is a new process developed with the rise of chip-on-board (COB) packaging technology. The basic principle is to introduce oxygen between Cu and ceramic, and then form Cu/O eutectic liquid phase at 1065 ~ 1083℃, and then react with ceramic base and copper foil to form CuAlO2 or Cu(AlO2)2 and realize the bonding between the copper foil and ceramic matrix under the action of the intermediate phase.

6. Vaccum Magnetron Sputtering

It is a kind of physical vapor deposition, which deposits multilayer film on the substrate by magnetic control technology， which has the advantages that other deposition technologies do not have, with better adhesion, less pollution, and improved crystallinity of the deposited sample to obtain high-quality film. The metallization layer obtained by this method is very thin, which can ensure the accuracy of the part's dimension. The DPC process supports PTH (electroplated through hole) /Vias (through hole). High-density assembly is possible - line/pitch (L/S) resolution can reach 20μm, thus achieving lightweight, miniaturization, and integration of devices.

Metallization Materials

❃ The Mo-Mn method mainly includes molybdenum, manganese, tungsten, nickel, silver, and gold.

❃ The DBC method mainly includes oxygen-free copper（OFC）

❃ Materials of other metallization methods: Palladium (Pd), Platinum (Pt), Titanium (Ti), and Aluminum (Al). Selected metal alloys may also be used.

Types of Metallized Ceramics

1. Metallized ceramic structural parts

They mainly protect, hermetic, support, insulation, connect, and dissipate heat. The main materials used include aluminum oxide (Al₂O₃) ceramic, zirconia toughened alumina (ZTA), Zirconia Ceramic (ZrO₂), aluminum nitride ceramic (AlN), beryllium oxide(BeO) and Boron nitride (BN).

2. Metallized Ceramic Substrate

In the application, it is mainly used as a circuit carrier to assist chip heat dissipation and insulation. The primary materials include alumina, aluminum nitride, Silicon Nitride , and beryllium oxide.

Uses of Metalized Ceramics

❃ High-power and high-frequency applications: power electronics, microwave devices, RF amplifiers

❃ Electronic components and devices: Integrated circuits, resistors and capacitors, sensors and transducers

❃ Hermetic packaging and sealing: vacuum tubes, electron tubes, optoelectronic devices, medical implants, and devices.

Conclusion

 The more profound research on the metallization mechanism of ceramics and the exploration and development of new processes is the basis for improving the metal and ceramic sealing surface, which will further expand the application field and is the future research direction.